Use Trusty for all builds except PHP 5.3

PHP 5.2 and PHP 5.3 are not supported on Travis CI:
"If you need PHP 5.2 or 5.3, please use Precise."

.travis.yml

@@ -1,5 +1,3 @@
-dist: trusty
-
 language: php
 
 matrix:
@@ -7,14 +5,23 @@ matrix:
     - php: nightly
   include:
     - php: 5.3
+      dist: precise
     - php: 5.4
+      dist: trusty
     - php: 5.5
+      dist: trusty
     - php: 5.6
+      dist: trusty
     - php: 7.0
+      dist: trusty
     - php: 7.1
+      dist: trusty
     - php: hhvm
+      dist: trusty
     - php: hhvm-nightly
+      dist: trusty
     - php: nightly
+      dist: trusty
 
 before_script:
   - bash tests/before_script.sh